8.1 Ptolemaic system

In his books On the Heavens , and Physics , Aristotle put forward his notion of an ordered universe or cosmos. It was governed by the concept ofplace , as opposed to space, and was divided into two distinct parts, the earthly or sublunary region, and the heavens. Theformer was the abode of change and corruption, where things came into being, grew, matured, decayed, and died; the latter was theregion of perfection, where there was no change. In the sublunary region, substances were made up of the four elements,earth, water, air, and fire. Earth was the heaviest, and its natural place was the center of the cosmos; for that reason theEarth was situated in the center of the cosmos. The natural places of water, air, and fire, were concentric spherical shellsaround the sphere of earth. Things were not arranged perfectly, and therefore areas of land protruded above the water. Objectssought the natural place of the element that predominated in them. Thus stones, in which earth predominated, move down to thecenter of the cosmos, and fire moves straight up. Natural motions were, then, radial, either down or up. The four elementsdiffered from each other only in their qualities. Thus, earth was cold and dry while air was warm and moist. Changing one orboth of its qualities, transmuted one element into another. Such transmutations were going on constantly, adding to the constantchange in this sublunary region.

The heavens, on the other hand, were made up of an entirely different substance, the aether

It is to be noted about this universe that everything had its natural place, a privileged location for bodies with aparticular makeup, and that the laws of nature were not the same in the heavenly and the earthly regions. Further, there were noempty places or vacua anywhere. Finally, it was finite: beyond the sphere of the fixed stars and the prime mover, there wasnothing, not even space. The cosmos encompassed all existence.